Monoethylene glycol (MEG) and monopropylene glycol (MPG) are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET. Ethylene and propylene glycols are typically made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
In recent years, increased efforts have focussed on producing chemicals, including glycols, from renewable feedstocks, such as sugar-based materials. The conversion of sugars to glycols can be seen as an efficient use of the starting materials with the oxygen atoms remaining intact in the desired product.
Current methods for the conversion of saccharides to glycols revolve around a hydrogenation/hydrogenolysis process as described in Angew. Chem. Int. Ed. 2008, 47, 8510-8513.
As with many chemical processes, the reaction product stream in these processes comprises a number of desired materials as well as diluents, by-products and other undesirable materials. During the hydrogenolysis of glucose and glucose-containing molecules, to form MEG and MPG, some of the glucose is hydrogenated to sorbitol and other sugar alcohols. Such sugar alcohols cannot undergo retro-aldol conversion to form MEG and MPG in the hydrogenolysis reaction. As a result, sorbitol and other sugar alcohols are side products and decrease the overall yield of the higher valued MEG and MPG.
In order to provide a high value process, the desirable product or products must be obtainable from the reaction product stream in high purity with a high percentage recovery of each product and with as low as possible use of energy, chemical components and complex equipment.
Sugar alcohols will generally be separated from the desired products as a heavies stream in one or more distillation steps. In certain processes, such heavies may be used as a convenient recycle solvent stream for a homogeneous hydrogenolysis catalyst. Thus, the presence of a certain amount of sugar alcohols in the reaction system is desirable. However, with more sugar alcohols being made during each reaction cycle, an undesirable build up can occur and removal of the sugar alcohols, e.g. via a bleed, is required.
Application U.S. application Ser. No. 62/367,384 describes a process for the conversion of sorbitol into ethylene glycol and propylene glycol by contacting a sorbitol feed with hydrogen in a reactor in the presence of a solvent and a bi-functional catalyst system. The sorbitol feed may be derived from a process for the conversion of saccharide-containing feedstock to MPG and MEG.
It would be desirable to provide a process that includes converting a portion of the sugar alcohols present in the product stream of a process for the conversion of saccharide-containing feedstock to MPG and MEG in-situ and in the presence of MEG and MPG, while retaining sufficient sugar alcohols in the product stream to allow efficient recycle of homogeneous catalyst materials.